Two-piece metal containers for food and beverages are often manufactured by drawing and wall ironing (DWI, also referred to as drawing and ironing (D&I)) or drawing and re-drawing (DRD) processes. The term “two-piece” refers to i) a cup-like can body and ii) a closure that would be subsequently fastened to the open end of the can body to form the container.
In a conventional DWI (D&I) process (such as illustrated in FIGS. 6 to 10 of U.S. Pat. No. 4,095,544), a flat (typically) circular blank stamped out from a roll of metal sheet is drawn though a drawing die, under the action of a punch, to form a shallow first stage cup. This initial drawing stage does not result in any intentional thinning of the blank. Thereafter, the cup, which is typically mounted on the end face of a close fitting punch or ram, is pushed through one or more annular wall-ironing dies for the purpose of effecting a reduction in thickness of the sidewall of the cup, thereby resulting in an elongation in the sidewall of the cup. By itself, the ironing process will not result in any change in the nominal diameter of the first stage cup.
FIG. 1 shows the distribution of metal in a container body resulting from a conventional DWI (D&I) process. FIG. 1 is illustrative only, and is not intended to be precisely to scale. Three regions are indicated in FIG. 1, where:                Region 1 represents the un-ironed material of the base. This remains approximately the same thickness as the ingoing gauge of the blank, i.e. it is not affected by the separate manufacturing operations of a conventional DWI process.        Region 2 represents the ironed mid-section of the sidewall. Its thickness (and thereby the amount of ironing required) is determined by the performance required for the container body.        Region 3 represents the ironed top-section of the sidewall. Typically in can making, this ironed top-section is around 50-75% of the thickness of the ingoing gauge.        
In a DRD process (such as illustrated in FIGS. 1 to 5 of U.S. Pat. No. 4,095,544), the same drawing technique is used to form the first stage cup. However, rather than employing an ironing process, the first stage cup is then subjected to one or more re-drawing operations which act to progressively reduce the diameter of the cup and thereby elongate the sidewall of the cup. By themselves, most conventional re-drawing operations are not intended to result in any change in thickness of the cup material. However, taking the example of container bodies manufactured from a typical DRD process, in practice there is typically some thickening at the top of the finished container body (of the order of 10% or more). This thickening is a natural effect of the re-drawing process and is explained by the compressive effect on the material when re-drawing from a cup of large diameter to one of smaller diameter.
Note that there are alternative known DRD processes which achieve a thickness reduction in the sidewall of the cup through use of small or compound radii draw dies to thin the sidewall by stretching in the draw and re-draw stages.
Alternatively, a combination of ironing and re-drawing may be used on the first stage cup, which thereby reduces both the cup's diameter and sidewall thickness. For example, in the field of the manufacture of two-piece metal containers (cans), the container body is typically made by drawing a blank into an intermediate, first stage cup and subjecting the cup to a number of re-drawing operations until arriving at a container body of the desired nominal diameter, then followed by ironing the sidewall to provide the desired sidewall thickness and height.
However, DWI (D&I) and DRD processes employed on a large commercial scale do not act to reduce the thickness (and therefore weight) of material in the base of the cup. In particular, drawing typically does not result in significant reduction in thickness of the object being drawn, and ironing only acts on the sidewalls of the cup. Essentially, for known DWI (D&I) and DRD processes for the manufacture of cups for two-piece containers, the thickness of the base remains relatively unchanged from that of the ingoing gauge of the blank. This can result in the base being far thicker than required for performance purposes.
Food, beverages, and other products are often packaged in two piece cans formed from aluminum, tin-plate steel, or coated steel sheets. Two piece cans include a can body having an integral base and sidewall and a lid that is seamed to the top of the sidewall of the can body.
Tin plate for can making typically is provided under ASTM A623 or ASTM A624 specifications. Even though most commercial tin plate is hot rolled or annealed late in the manufacturing process, often a surface cold rolling process provides an identifiable grain direction. The grains in commercial tin plate for can making are not equiaxed, but rather in a cross sectional sample define a longitudinal direction, which defines the grain direction, and a transverse direction. The grains boundaries are visible upon magnification by widely accepted techniques, such as described in ASTM E 112.
Aluminum for canmaking often begins as a sheet of 3104-H19 or 3004-H19 aluminum alloy, which is aluminum with approximately 1% manganese and 1% magnesium for strength and formability. The cold rolling process used to produce commercial grade aluminum for canmaking yields a metal sheet having non-equiaxed grain structures. In this regard, aluminum sheet grains define a longitudinal direction and a transverse direction. Because of the amount of cold rolling, grains in commercial aluminum sheet for can making are elongated compared to grains in commercial tinplate for canmaking.
There is a need for improved can technology and improved cans that make efficient and effective use of sheet material that takes advantage of economics of metal supply.